This invention relates to biodegradable resin which has been recently spotlighted in place of synthetic resin, and more particularly to a biodegradable resin foam obtained by foaming the biodegradable resin and a method and an apparatus for producing the same.
In general, synthetic resin has been applied to a variety of industrial fields because of exhibiting satisfactory mass productivity, moldability and durability. In particularly, a synthetic resin foam is light-weight and exhibits increased cushioning properties, to thereby be widely used in various forms such as a protective casing for a fragile article such as a glass product, a cushioning material for packing, a tableware, a heat insulation material, a sound insulation material and the like. However, this causes the amount of disposal of such synthetic resin products to be extensively increased, leading to various serious problems.
More particularly, incineration of synthetic resin causes a large amount of harmful gas to be produced, leading to atmospheric pollution. Disposal of synthetic resin other than the incineration causes environmental pollution because it has resistance to oxidation and resistance to decomposition by light and ozone. Also, synthetic resin is extensively increased in intermolecular bond, so that the incineration causes generation of much heat, leading to damage to an incineration furnace and therefore a decrease in lifetime of the furnace.
In view of the foregoing, much attention has been recently directed to biodegradable resin and a great effort has been made to develop biodegradable resin.
As a result, processing of biodegradable resin into a film material is now in the course of being put into practice. Also, development of foaming of biodegradable resin would lead to spread of applications thereof, to thereby permit advantages of biodegradable resin to be more widely exhibited. Techniques of foaming synthetic resin which have been carried out in the art include a method of producing foamed beads including the steps of charging styrene beads in a forming mold and adding water vapor thereto, followed by a decrease in pressure, a method of foaming synthetic resin by charging an extruder with, for example, styrene resin together with a foaming agent such as an organic solvent or the like to foam the resin due to a pressure reducing action occurring when the resin is extruded, and the like.
However, such conventional chemical foaming techniques for foaming synthetic resin as described above fail to satisfactorily foam biodegradable resin due to a relationship between a softening point or melting point of the resin and a foaming temperature of a foaming agent and the like. Thus, there are known many problems which are encountered with techniques of foaming biodegradable resin to a high degree and forming the foamed resin.
A first problem occurs when a biodegradable resin foam is to be produced by means of, for example, an injection molding machine used for production of a conventional synthetic resin foam. More particularly, when biodegradable resin fluidized due to heating and pressurization in a cylinder is extruded through a nozzle of the cylinder into a forming mold, to thereby be decreased in pressure, moisture in the resin is vaporized, leading to expansion. The moisture vaporized is then decreased in temperature, resulting in suspending in the form of steam in the mold or being condensed on an inner surface of the mold or an outer surface of a molded product. Biodegradable resin generally exhibits increased hygroscopicity, resulting in being readily softened and swollen when it is contacted wit moisture. In particular, a film of each of foamed cells on an outer portion of a molded or formed product is excessively decreased in thickness, so that it is readily softened when it absorbs condensed water. This results in the foamed cells being readily collapsed. Such collapse of the cells is also caused due to re-adhesion of moisture evaporated from the resin to the cells. The collapse causes portions of the formed product at which the cells are collapsed to be shrunk, leading to deformation of the formed product. When the formed product thus deformed is solidified, it is caused to be in a solid form substantially free of any foamed cell. Thus, the formed product thus obtained by injection molding fails to exhibit desired cushioning performance.
A second problem is that the conventional chemical foaming techniques fail to provide a foamed product having a desired configuration and exhibiting a satisfactory cushioning function. More particularly, foaming of biodegradable resin is started upon release from a pressurized state, however, it is highly hard to reach the depths of a forming mold because of exhibiting increased viscosity when it is fluidized by heating. Therefore, foaming of biodegradable resin partially occurs before the resin extruded from the cylinder reaches the depths of the forming mold, resulting in a part of the resin which is to be foamed in the depths of the mold carrying out foaming in the middle of the mold, so that any cavity and/or void are formed in the foamed resin. Such a problem tends to occur in a forming mold of a complicated configuration, Thus, the so-formed biodegradable resin foam is pressedly forced by biodegradable resin subsequently extruded, so that the portion of the resin which carried out foaming on the way to the depths of the mold is crushed by the subsequently extruded resin. Thus, the prior art fails to form the foamed resin into a desired configuration. Also, the foamed resin fails to exhibit a satisfactory cushioning performance.
A third problem occurs due to releasing of biodegradable resin fluidized by heating and pressurizing from a heated and pressurized environment. Releasing of the resin fluidized causes moisture contained in the resin to be vaporized and expanded, resulting in foaming of the resin, to thereby provide cells, during which the cells are decreased in temperature to a level of about 100.degree. C. due to vaporization of the moisture. This causes the cells to be somewhat shrunk and then solidified while being kept shrunk. Also, the cells are somewhat shrunk by water vapor surrounding the cells. Such cells are integrated together to form a foam. Thus, cavities and/or voids occur in the foam solidified, so that boundaries between the cells are discontinuous, to thereby cause the foam to be unsuitable for use for a cushioning material.
A fourth problem encountered with the conventional chemical foaming techniques is caused during formation of an pressure reduced atmosphere. More particularly, when a heated and pressurized atmosphere in which biodegradable resin is placed is to be changed into a pressure reduced atmosphere, an evacuation or vacuum pump is generally used. Unfortunately, formation of such a pressure reduced atmosphere requires a considerable period of time, so that a pressure reducing action due to the change is rendered slow or inactive. Also, this causes moisture to re-adhere to cells while it is not fully exhausted, leading to softening of the cells, followed by collapse of the cells, resulting in the resultant resin foam being deteriorated in properties or quality. In order to prevent such re-adhesion of moisture to the cells, it would be considered that formation of the pressure reduced atmosphere is carried out using a large-sized vacuum pump or the like, resulting in time required for evacuation being reduced. However, this causes a significant increase in manufacturing cost of the foam.
Further, injection of biodegradable resin from a nozzle of a cylinder of an injection molding machine into a forming mold arranged in a closed atmosphere causes injection resistance to be increased. In order to avoid the problem, it is required to arrange a large-sized injection molding machine. Unfortunately, this leads to an increase in cost of equipment and therefore manufacturing cost. In particular, in order to form a foam with high configuration accuracy, it is required to permit biodegradable resin to be spread throughout the forming mold. This is advantageously accomplished by keeping an atmosphere in the forming mold pressurized during injection molding. However, this causes injection resistance to be further increased, resulting in the above-described disadvantage being rendered amplified.